1. Field of the Invention
The invention relates to the positioning of a glass sheet traveling on a conveyor, particularly a glass sheet heated to beyond its softening point with a view to its cambering and/or a heat treatment for modifying certain of its mechanical characteristics. The invention more particularly applies to the production of glass plates for motor vehicles.
2. Background of the Related Art
In order to supply a glass sheet with good conformity of camber and a satisfactory optical quality, it is important that the glass sheet is correctly positioned prior to being taken up by the cambering and/or heat treatment tools. However, a glass sheet heated to beyond its softening point cannot be held by grippers or other gripping members, unless one is willing to accept the marks necessarily left behind by them. It is also necessary to position to glass sheet relative to the tools prior to the heating thereof, i.e. the tools must be stopped for an extremely long time, which leads to very high productions costs.
These problems of gripper marks and the stopping of tools are reduced in the case of cambering and/or heat treatment processes in which the first stage, namely the heating of the glass sheet, takes place in a furnace traversed by the glass sheets, e.g. on a roller bed. However, the positioning problem is not solved, because the travel speeds and lengths of the furnaces are such that two successive glass sheets do not always have strictly the same trajectories. These trajectory differences can lead to two types of faults, namely sliding perpendicular to the axis of the conveyor and rotation with respect to said axis and which are respectively obviated by lateral positioning means and frontal positioning means.
As stated in European Patent Application No. EP-A-267 120, lateral positioning may be obtained by means of guide bars which move together while advancing in the forward movement direction of the conveyor and which define a passage for the glass sheet. The glass rubs against these bars, but is not stopped by them.
However, as glass sheets intended for motor vehicle windows are never formed by rectangular plates and instead most always have a length varying from one end to the other of the window, said lateral positioning is not adequate and must be completed by a frontal positioning which, according to EP-A-No. 267 120, involves a glass sheet stopping phase. In order to give the glass sheet the desired orientation, a stop means is positioned in its path and which may be one constituted by two abutments, which are necessarily struck by one end of the leading edge of the sheet and maintained in place for an adequate period of time to enable the glass to be reoriented by the conveyor continuing to move the glass sheet. For as long as the glass sheet is at least partly stopped by the stop means, it always rubs at the same point on the rollers, which can then leave behind an impression on the glass which, due to its high temperature, is very easily marked. Therefore the frontal positioning is the source of numerous optical defects.